Thermomechanical Fatigue Behavior of the High-Temperature Titanium Alloy IMI 834

The isothermal and thermomechanical fatigue (TMF) behavior of the titanium alloy IMI 834 was studied between 350 °C and 650 °C in air and vacuum, respectively. Transmission electron microscopy (TEM) observations revealed that the microstructure established in the TMF tests was governed by the maximum temperature within the cycle. However, if the maximum temperature does not exceed 600 °C, planar dislocation slip prevails and similar microstructures are formed regardless of the test temperature and the testing mode (TMF and isothermal, respectively). As a result, the stressstrain response in TMF tests can be assessed from the corresponding isothermal data. Wavy dislocation slip was found to determine the stress-strain behavior if the maximum test temperature exceeded 600 °C. Moreover, in TMF tests with a maximum test temperature of 650 °C, the dislocation arrangement formed in the high-temperature part of the hysteresis loop was found to be stable throughout the cycle and to affect significantly the stress-strain response at the low temperatures. Although in-phase (IP) and out-of-phase (OP) TMF tests led to an almost identical microstructure, OP loading was always found to be most detrimental. The interaction between the embrittled subsurface layer, caused by oxygen uptake, and the high tensile stresses developing in the low-temperature part of the hysteresis loop in OP tests eases crack initiation and initial crack propagation and results in reduced fatigue life.